H 2 S as end-product of the reaction. As mentioned earlier, denitrification is
only effective under oxygen depletion. In this situation, nitrate may be
reduced to gaseous nitrogen (N 2 ) by several steps involving nitrite and nitric
oxide. Nitrogen is released to the atmosphere. Although this process is disad-
vantageous for agricultural purposes, it may be of great use for sewage treat-
ment and also in treatment of stone and wall paintings.
Since both processes only take place under oxygen depletion, they have to
be carried out under oxygen-free conditions on site using compresses with
airtight coverings. Sulfate-reducing bacteria cannot use oxygen and their activ-
ity is thereby restricted to the application phase. Denitrifying bacteria, how-
ever, may also be active under aerobic conditions and may then contribute to
biodeterioration processes. To overcome these problems, attempts have been
made to apply the bacteria in an immobilised form. This not only guarantees
complete removal of cells after application, but also minimises the problem of
biodeterioration afterwards. However, the activity of these immobilised bacteria
is restricted to the surface of the objects, where they are in direct contact with
the porous materials.
Both techniques are more or less still under investigation in preliminary appli-
cations. However, they may be an interesting alternative in the future, if exe-
cuted with immobilised bacteria, because complete removal of potentially
detrimental bacteria by cleaning methods after the treatment is difficult to
achieve. Even if biocides were applied for disinfestation purposes, the cell
material (biomass) would stay on the surface and in the material pores and
provide a nutrient supply for other microorganisms.
3.2 Deterioration by Microorganisms
Factors enhancing microbial colonisation. The essential precondition for
microbial colonisation and possible deterioration is the adequate supply of
moisture. The required amount may vary considerably for different microor-
ganisms. Even among the commonly encountered species of moulds, there are
those that grow in a rather low water environment (*awvalues from 0.75) but
also species only growing at a high water activity at 0.95. In general it is
considered that fungi can grow at rather low water levels, while bacteria and
algae grow at higher moisture. It must be stressed that moisture is the most
essential factor formicrobial colonisation and even if all other demands con-
cerning the habitat are met, there will be no microbial colonisation without
sufficient moisture. Nevertheless, more preconditions have to be met. For the
formation of a microbial community, a range of nutrients have to be available.
Wall Paintings: Aspects of Deterioration and Restoration 247
*awis water activity where aw , a measure of available water on a scale 0–1; pure water
has an awof 1 and the lowest awfor biological activity is 0.6.p(material)
p 0 (pure water)